1. Field of the Invention
The invention concerns a device for handling thin panes of glass or similar work pieces and products in conveyor systems, machining installations, and the like by pneumatic forces.
2. Summary of the Related Art
Thin plate glass is used on a large scale to produce displays, where scratches, inclusions, deposits, etc. larger than approximately 10 .mu.m are seen as defects because they cause intolerable local distortion of the image in the finished display. Therefore, extensive control measures must be provided to detect and reject products that cannot be used.
Since there are also high demands regarding the flatness and parallelism of the plate glass used for window displays, the manufacturing process usually includes a surface polishing step. This makes it possible to eliminate many defects but it makes production considerably more expensive. There have therefore been great attempts to eliminate the polishing operation.
In recent plate glass manufacturing processes, the demands for flatness and parallelism of the glass surfaces can be maintained to a better extent on drawn or floated raw glass, so that polishing can be eliminated because of the cost advantages. However, this also eliminates the possibility of eliminating defects. On the other hand, plate glass in the thickness range of 30 to 300 .mu.m for small display formats cannot currently be polished or it can be polished only at a high cost. Current projections indicate that formats up to 650.times.1000 mm with a plate glass thickness of 50 .mu.m can be expected. In addition, the specifications with regard to the size of defects and the number of defects per unit of area have become increasingly more strict with the newer display grades of glass, e.g., field emission display (FED), plasma display panel (PDP), plasma addressed liquid crystal (PALC), thin film transistor (TFT) or super twisted nemetic (STN), so that glass manufacturers are forced to comply with stricter requirements to be able to offer a technically and economically acceptable product.
An indispensable manufacturing step is formatting the raw glass, which is manufactured in large plates or as a wide strip for economic reasons. Formatting therefore includes cutting to the desired display size, including processing of edges and comers, which may optionally be followed by a final cleaning.
A device for conveying glass in the form of a plate or a strip is already known from German Patent DE 1,756,070 B2, where the glass rests on a cushion of gas at a distance from a lower structure and can be conveyed horizontally relative to the lower structure without coming in contact with it. The lower structure is provided with slots side by side for the passage of the gas, although the slots are designed alternately as gas discharge and exhaust slots and are connected to appropriate pressure and vacuum sources. This generates an alternating field of mutually opposed forces which a thin pane of glass cannot withstand. Furthermore, only one of the two glass surfaces, at best, can be protected from soiling.
Furthermore, European Patent EP 336,332 B1 discloses a device for gripping, stacking and unstacking plate glass as well as conveying it, where vacuum grippers, which are known from another technology, are used. However, suction grippers cannot be used with glass below a certain minimum thickness because thin plate glass would sag in the area of the vacuum grippers or could even break in the worst case. Furthermore, vacuum grippers are not suitable for holding very thin plate glass for machining such as cutting, grinding and/or polishing because they result in uneven gripping, which cannot be maintained uniformly in the machining area. Moreover, the vacuum grippers do not prevent soiling of the glass surfaces in conveyance and machining.